Subject: Returned mail: see transcript for details

From: Mail Delivery Subsystem <[email-masked]>
Date: Thu, 9 Aug 2007 11:05:37 -0400 (EDT)

The original message was received at Thu, 9 Aug 2007 11:05:32 -0400 (EDT)
from exprod6mx177.postini.com [64.18.1.68]

   ----- The following addresses had permanent fatal errors -----
<[email-masked]>
    (reason: 550 [email-masked] unknown user account)

   ----- Transcript of session follows -----
... while talking to mail.corp.idt.net.:
>>> DATA
<<< 550 [email-masked] unknown user account
550 5.1.1 <[email-masked]>... User unknown
<<< 554 no valid RCPT address specified